In a cathode material used in a secondary battery such as a lithium metal battery, lithium ion battery, or lithium polymer battery, for example, a metal oxide, an oxide obtained from the metal oxide by partially substituting one or more metallic atoms therein, a phosphate such as LiFePO4 and LiCoPO4, or a sulfate such as Fe2(SO4)3, an electrode redox reaction proceeds in the course of discharging or charging in such a way as accompanied by doping/undoping of ions of an alkali metal such as lithium. Recently, such secondary batteries are attracting considerable attention as large capacity batteries. In the cathode of such a battery, however, the velocity of alkali metal ions moving in the electrode material by solid-phase diffusion governs the rate of the electrode reaction, and therefore, substantial polarization generally takes place in the electrode reaction during charging or discharging, thereby making it difficult to charge or discharge at a relatively large current density. When this polarization is especially pronounced, the charging or discharging does not go on sufficiently under usual voltage and current density conditions, so that the secondary battery can be used at a substantially smaller capacity as compared to the theoretical capacity. Further, metal oxides, phosphates, sulfates, metal oxo-acid salts and the like, which are commonly used as such cathode materials, generally have low conductivity, and therefore, also act as a cause of increased polarization in the electrode reaction.
To alleviate the problems described above, it is effective to control the crystal grains of each cathode material to fine sizes so that alkali metal ions can easily move into and out of the crystal grains. The control of the crystal grains to such fine sizes leads to an improvement in conductivity because the contact area between a conductivity-imparting material commonly mixed with the cathode material, such as carbon black, and the cathode material increases, and as a result, the polarization in the cathode reaction can be reduced while making improvements in voltage efficiency and specific battery capacity.
Recently there have been made some reports on attempts conducted to achieve the above-described object. In attempts to obtain a cathode material with small grain size, the crystal growth of the cathode material was suppressed by using raw materials of high reactivity to lower the calcination temperature and at the same time, limiting the calcination time upon synthesizing the cathode material by calcination. For instance, there is a report that, upon production of LiFePO4 as a cathode material for a lithium secondary battery, LiOH.H2O of high reactivity was used as a source of lithium, calcination was carried out in argon for a relatively short time (about 24 hours) at 675° C.—which is lower than temperatures employed in the conventional technology (generally from 800 to 900° C. or so)—to inhibit sintering (an increase in the grain size) of the cathode material powder and as a result, a large discharge capacity was obtained (The 40th Battery Symposium, Report 3C14 (Preprint, p349, 1999); The Electrochemical Society of Japan).
Jp 2001-15111 A does not disclose any method for suppressing the crystal growth of an electrode material, but discloses a process for depositing carbon on surfaces of grains of a complex oxide (including a metal oxo-acid salt such as a sulfate, a phosphate, or a silicate) represented by the chemical formula of AaMmZzOoNnFf wherein A indicates an alkali metal; M indicates Fe, Mn, V, Ti, Mo, Nb, W or any other transition metal; Z indicates S, Se, P, As, Si, Ge, B, Sn or any other non-metal) to raise the surface conductivity. It further discloses that use of such composite material in the electrode systems of a battery or the like makes even and stable the electric fields around interfaces among grains of the complex oxide, a current collecting (conductivity-imparting) material and an electrolyte in the course of electrode redox reaction. As a procedure for depositing carbon on the surfaces of the grains of the complex oxide, this publication propose to make an organic material, which deposits carbon through pyrolysis (such as a polymer, monomer, or low molecular weight compound), exist together or to add carbon monoxide and then subject it to pyrolysis (the publication also disclose that the composite material of the complex oxide and surface-covering carbon can also be obtained by making the organic material exist together with raw materials for the complex oxide and thermally subjecting them to reactions at once under reducing conditions). In Jp2001-15111 A, the above described process and procedure realize an improvement in the conductivities of the surfaces of the complex oxide grains as mentioned above, and achieve high electrode performance such as high discharge capacity, for instance, when a Li polymer battery is formed with a composite material prepared by depositing carbon on the surfaces of grains of a cathode material such as LiFePO4.
With an approach employing a lower temperature and/or a shorter time for calcination upon synthesizing a cathode material by calcination like the process described in Report 3C14read at The 40th, Battery Symposium (Preprint, p349, 1999), calcination may not be performed sufficiently in some instances so that chemical changes may not proceed to give the final product or intermediate products may remain in the final product. This approach, therefore, has a limit as a method for controlling the grains of a cathode material to fine sizes.
The method disclosed in Jp 2001-15111 A is effective for improving the surface conductivity of an electrode material. However, it makes no mention whatsoever about the inhibition of crystal growth during the synthesis of the electrode material, and further, it does not contain any disclosure about a method for controlling the deposition of carbon on an electrode material more advantageously from the standpoint of the performance of electrodes.
For the reasons described above, there is an outstanding desire for the provision of a novel method for producing a cathode material for secondary batteries, which can assure the synthesis of a desired cathode material from raw materials by calcination and can also inhibit crystal growth of primary grains of the cathode material to control their sizes fine and to impart excellent conductivity. There is another outstanding desire for the provision of a high-performance secondary battery with improved voltage efficiency and specific battery capacity by optimally controlling grains of a cathode material to fine sizes and imparting conductivity to promote movements of ions of an alkali metal led by lithium between the interiors of grains of the cathode material and an electrolyte such that the polarization in the electrode reactions is suppressed and the contact area between the cathode material and the conductivity-imparting materials is increased to improve the conductivity.